(Reuters) - Drugmaker Aerie Pharmaceuticals Inc (AERI.O) said on Wednesday its experimental eye drop met the main goal of a late-stage study, sending its shares up as much as 65 percent in extended trading.

The drug, Rhopressa, showed that it was not inferior to a commonly prescribed treatment, timolol, in reducing pressure inside the eye in patients suffering from glaucoma, or ocular hypertension, the company said.

Increased pressure in the eye is often a sign of glaucoma, a condition that could lead to blindness if left untreated.

Results from an earlier late-stage study had prompted the company to change the main goal of the second study for Rhopressa be as effective as timolol in patients whose eye pressure was between 20-25 millimeters of mercury (mmHg).

In April, Aerie Pharma said Rhopressa was as effective as timolol in patients whose eye pressure was between 20 and 26 mmHg.

But the main goal was for the drug to be non-inferior to timolol in patients whose eye pressure was as much as 27 mmHg.

About 80 percent of glaucoma patients have eye pressure of 26 mmHg or less at the time of diagnosis.

Rhopressa is estimated to rake in sales of over a billion on approval.

Aerie's shares were trading at $29.70 after the bell on Wednesday.

(Reporting by Vidya L Nathan in Bengaluru; Editing by Sriraj Kalluvila)